1. Field of the Invention:
The invention relates to an improved threaded connection for oil well pipes manufactured in accordance with A.P.I. specifications, and to a method for fabricating such threaded connection.
2. Summary of the Prior Art:
Substantially all pipe utilized in oil wells, and particularly conduit elements subject to substantial amounts of tension when installed in the well, such as casing pipe, are fabricated to comply with American Petroleum Institute (A.P.I.) specifications. Such specifications involve a significant range of dimensional tolerances both for the internal diameter of the pipe and the external diameter. Many oil well drillers prefer that the threaded connection between the successive lengths of pipe be of the so-called flush configuration which inherently limits the amount of metal available for cutting threads on the pin end of one pipe and in the box end of the connecting pipe.
To improve the efficiency of such threaded connections, and particularly the resistance of the threaded connections to applied tension, so-called premium threads have been developed, such as the BAKER RSS Premium Thread currently sold by BAKER TUBULAR SERVICES, INC. of Houston, Texas. Such premium threads involve so-called two step threads, in other words two sets of axially adjacent threads with the outermost set on the pin being of substantially smaller diameter than the innermost set of threads on the pin. Metal seals are provided by conical surfaces at opposite ends of the threaded connection and it is common to provide a reverse angle torque stop surface intermediate the two axially adjacent threads.
When such threads are fabricated by conventional machining practices, a substantial variation in the tension resistance of the threaded joint was observed. Extensive tests uncovered the fact that the variance was caused by the A.P.I. tolerance on the OD and ID of the pipe. These variations as allowed by A.P.I. tolerances, could produce as much as a twelve percent reduction in the critical areas of the two step threaded connections. Thus, random failures were repeatedly encountered in the field when conventionally formed two step threads were employed in pipe strings requiring substantial tensile strength of the threaded joints, preferably a minimum of seventy percent of the ultimate tensile strength of the pipe wall. Accordingly, there is room for substantial improvement in the reliability of threaded connections for effecting two step flush joints of pipes manufactured in accordance with A.P.I. specifications.